"We can't know the mind of God" is a popular theistic smokescreen that utterly fails upon examination.
In several discussions with theists, particularly when the problem of natural evil is the topic of discussion, one of the last defenses of theists, when all of their arguments are seemingly defeated, is "we can't know the mind of God." In a longer form, this typically means something similar to "Our knowledge, when reflecting on what an omniscient being can and does know, is so, so, so slim. To claim that God has no reason for allowing something to happen is so arrogant. How can you claim to know the mind of God? Perhaps God has some reasons that we are unaware of that can justify all of this evil."
In a debate with philosopher Stephen Law, William Lane Craig used this as one of his defenses for the problem of evil. This view is quite problematic, ultimately fails to offer a solid defense to the problem of evil, gives us no new knowledge, and seems to be quite dishonest in the case of the many other claims Christians make about the supernatural.
Claiming that one "can't know the mind of God," especially when the problem of natural evil is being discussed, when using the "We can't know the mind of God" defense, typically is asserting that there may be some good reasons that God has for allowing evil...so this suffering now really isn't, perhaps, so much of a bad thing. Going farther, some may even assert that an afterlife 'make up for' the suffering now.
The main problem with these above views is that the theist is forced into an odd kind of moral skepticism. If some event x may lead to a greater good 'for all we know,' can we really say that some event x is really a bad thing? How about 'good' events? 'For all we know,' 'good' event y might not be conducive to good and our perceptions might be wrong! You can't know the mind of God, after all! If we can't really know the mind of God when 'evil' is concerned, it seems to follow that we also can't know the mind of God when 'good' is concerned. That earthquake over there...who are you to say that that is a bad thing? Perhaps it's part of God's plan and this earthquake is needed for the salvation of those two month children! As you see, this "you can't know the mind of God" defense leads the theist into making absurd statements.
The "we can't know the mind of God" defense also seems to go against everything we can possibly know about ethics and seems to throw all moral progress to the wayside. If "we can't know the mind of God," clearly all moral philosophy is a waste of time...and everything we can claim about ethics is flawed because, for all we know, what we say is undesirable might actually be desirable. As hard as we try, it seems, "if we can't know the mind of God," we can't ever confidently make tenable claims about ethics. Again, the theistic defense is patently absurd.
Further, "we can't know the mind of God" seems quite contrary to what theists typically claim. They will say that adultery, murder, rape, killing children for the fun of it, and so much more are undesirable. If it is the case that we can't know the mind of God, how can we possibly say that we do know the mind of God when certain issues are concerned? Why is it the case that we can know the mind of God when some issues are concerned and such issues, without even appealing to God, seem quite evident...and other issues, such as the egregious suffering of animals and humans lead theists to say, even though these issues are quite evident (and seem to lead us to the conclusion that an omni-good god would not 'design' this plan), that "we can't know the mind of God?" It seems, rather, that theists are trying to rationalize their beliefs; instead of saying "I have no answer for this and it really seems that there is no all-loving god," they say "we can't know the mind of God."
Saying "we can't know the mind of God" seems to be an egregious ad hoc defense that renders belief in God unfalsifiable, thus making belief in God irrational. For an analogy of sorts, suppose I claim that there is a unicorn in my room. You look inside and do not see such a unicorn and have good reasons, before even stepping into my room, that there are no good reasons to believe in unicorns because all defenses you have heard were quite lackluster. When you try to find the unicorn, I say things like the unicorn is invisible, shy, incorporeal, immune to detection, etc. You'd be justified in believing that my belief was irrational because nothing you could say, if you listen to my defenses, could ever serve to show that the belief is false. Much in the same way that the unicorn belief is irrational, so is the belief in God if the believer says "We can't know the mind of God" because every attempt the non-believer tries to show that the belief is irrational is simply 'excused away' and no possible argument can serve as a defeater.
Claiming "we can't know the mind of God" adds nothing whatsoever to our knowledge and thus should be rejected as an explanation. In his article "Can God Explain Anything," in Think, philosopher Theodore Shick talks about explanatory power as being a very important criterion to consider when evaluating a hypothesis. He writes, "We seek explanations because we seek understanding. The best explanation, then, is the one that produces the most understanding. The amount of understanding produced by an explanation is determined by how well it systematizes and unifies our knowledge." It seems to follow, then, that "we can't know the mind of God" is no good defense [for the problem of evil] because it provides no knowledge.
When we consider knowledge, hopefully, we understand that our claims ought to be the best representations of reality. While we certainly can be wrong about everything, all we can possibly work with is what we know now. If we were to globally apply "we can't know [the mind of God]," we would have to be agnostic about everything, it seems. For all we know, of course, we could all be part of a computer simulation and the world as we know it is an advanced program run by someone in an alternate universe. We have no good reason to believe this, so we simply don't. Shall we also be agnostic about our moral beliefs? Of course not....we have no good reason to.
"We can't know the mind of God" also falls prey to other problems. Stephen Law, in his article "The God of Eth," among other things, mentions that the standard defenses for the problem of evil can be 'mirrored' to defend belief in an evil god...and theists, of course, recognize that "you can't know the mind of an all-evil god" fails and is profoundly irrational. Of course theists don't believe in an omni-good god or an omni-evil god based on evil or good in the world, as critics might contend, but this misses the point of Law's argument. Theists can typically offer arguments for a god, but they seem to get no further, if we even are to 'accept' them, than a deistic god or a 'vanilla' god with no moral designation. Even if a god allows us to know what us objectively good and we accept the idea of objective moral values, we still can't conclude that this god is good - perhaps the god is neutral or evil and we can know regardless. If theists won't accept "you can't know the mind of an omni-evil god," we do they use "we can't know the mind of an omni-good god?"
"You can't know the mind of God" seems to lead a theist into utter moral skepticism, asserts that our notions of right and wrong are untenable, seems contrary to theistic claims of what they do know, renders theistic belief to be unfalsifiable [thus making it irrational], gives us no new knowledge, and is contrary to how we typically consider knowledge [based on what we know now]. Instead of offering poor defenses to the problem of evil like "you can't know the mind of god," theists should admit that there is no good defense for the problem of evil and realize that their belief in an omni-good god is irrational. The problem of evil is unanswered and serves as a defeater to belief in an omni-good god.